Aluminum nitride (AlN) holds great promise as a semiconductor material for numerous applications, e.g., optoelectronic devices such as short-wavelength light-emitting diodes (LEDs) and lasers, dielectric layers in optical storage media, electronic substrates, and chip carriers where high thermal conductivity is essential, among many others. In principle, the properties of AlN may allow light emission at wavelengths down to around 200 nanometers (nm) to be achieved. Recent work has demonstrated that ultraviolet (UV) LEDs have superior performance when fabricated on low-defect AlN substrates prepared from bulk AlN single crystals. The use of AlN substrates is also expected to improve high-power radio-frequency (RF) devices made with nitride semiconductors due to the high thermal conductivity with low electrical conductivity. However, the commercial feasibility of AlN-based semiconductor devices is limited by the scarcity and high cost of large, low-defect single crystals of AlN.
To make large-diameter AlN substrates more readily available and cost-effective, and to make the devices built thereon commercially feasible, it is desirable to grow large-diameter (>25 mm) AlN bulk crystals at a high growth rate (>0.5 mm/hr) while preserving crystal quality. The most effective method of growing AlN bulk single crystals is the “sublimation-recondensation” method that involves sublimation of lower-quality (typically polycrystalline) AlN source material and recondensation of the resulting vapor to form the single-crystal AlN. U.S. Pat. No. 6,770,135 (the '135 patent), U.S. Pat. No. 7,638,346 (the '346 patent), U.S. Pat. No. 7,776,153 (the '153 patent), and U.S. Pat. No. 9,028,612 (the '612 patent), the entire disclosures of which are incorporated by reference herein, describe various aspects of sublimation-recondensation growth of AlN, both seeded and unseeded.
While AlN substrates are enabling platforms for the fabrication of UV light-emitting devices such as LEDs, their performance in such applications is often limited by their transparency to UV light (i.e., “UV transparency”) or lack thereof. AlN substrates with high UV transparency are often difficult to produce, as UV transparency is compromised by contamination and/or point defects introduced during the AlN growth process. Such issues have been addressed on a limited basis via techniques disclosed in U.S. Pat. Nos. 8,012,257, 9,034,103, and 9,447,519, the entire disclosure of each of which is incorporated herein by reference. Specifically, these patents disclose techniques for controlling the introduction of oxygen impurities during polycrystalline AlN source-material preparation and sublimation-recondensation growth of single-crystal AlN. While such techniques were reported as enabling production of bulk AlN crystals having low absorption coefficients, and thus high UV transparency, it has been found by the present inventors that such techniques are incapable of producing high UV transparency when utilized in conjunction with seeded growth of AlN bulk crystals exceeding 25 mm in diameter (e.g., crystals between 30 mm and 75 mm in diameter, for example approximately 50 mm in diameter) at high growth rates (e.g., at least 0.5-0.8 mm/hr) and utilizing the large axial and radial thermal gradients necessary for such growth.
For example, FIG. 6 of U.S. Pat. No. 9,447,519 (the '519 patent) depicts an absorption spectrum of an AlN crystal produced utilizing the oxygen-control techniques and controlled post-growth cooling described in the '519 patent. As the figure indicates, that AlN crystal had an absorption coefficient below about 10 cm−1 for the wavelength range between 300 nm and 350 nm. However, that crystal was produced utilizing unseeded growth and had a maximum diameter of less than 25 mm. The present inventors attempted to reproduce this high UV transparency utilizing the same techniques for an otherwise substantially identical growth process involving seeded growth of a crystal of approximately 50 mm in diameter. Unfortunately, even utilizing the techniques of the '519 patent, the resulting crystals were effectively opaque, i.e., exhibiting an absorption coefficient greater than 100-200 cm−1 over one or more UV wavelengths, and/or exhibiting large peaks in the absorption coefficient at approximately 265 nm and/or 310 nm. An example graph of the absorption coefficients of different wafers sliced from a boule produced using the techniques of the '519 patent and having a diameter of 50 mm is shown in FIG. 1A. As shown, each of the wafers from this boule is essentially opaque at various UV wavelengths. The UV transparency of similar crystals was particularly poor for crystals produced utilizing an on-axis AlN seed. On-axis growth is preferred for economic reasons; off-axis boules of AlN must be cut at an angle to produce on-axis substrates therefrom, and thus the number of substrates that may be produced from an off-axis boule is necessarily less than that which may be produced from an on-axis boule. In view of these results, the present inventors recognized a need for novel and improved techniques of achieving high levels of UV transparency in larger AlN bulk crystals.